Heat Treatment Effects on GCr15 Cold-Rolling Mill Roll Cladding Microstructure and Wear Resistance Study Note

Literature Overview and Background

This study investigates the influence of post-weld heat treatment on the microstructure and wear resistance of GCr15 (Chinese standard bearing steel, equivalent to AISI 52100) cold-rolling mill roll cladding layers. Cold-rolling mill rolls are subjected to extreme contact stresses (up to 4 GPa), sliding friction, and thermal cycling during operation. The GCr15 cladding layer, typically deposited by high-speed burring (HVOF) or plasma spraying, must exhibit high hardness, fatigue resistance, and dimensional stability.

Core Technical Points

Microstructural Response to Heat Treatment

GCr15 is a high-carbon chromium bearing steel (0.95-1.05% C, 1.40-1.65% Cr) that can achieve hardness of 62-66 HRC after proper heat treatment. The as-deposited cladding layer typically exhibits a martensitic structure with retained austenite (10-25%), depending on the deposition process. The study examines how different tempering temperatures and holding times affect the microstructure and wear performance.

Heat Treatment Condition	Hardness (HRC)	Retained Austenite (%)	Microstructure
As-deposited (HVOF)	58-62	15-20	Fine martensite + retained austenite
Tempering at 150 °C / 2h	60-64	12-15	Tempered martensite + fine carbides
Tempering at 200 °C / 2h	58-62	10-12	Tempered martensite + Cr7C3
Tempering at 250 °C / 2h	55-58	8-10	Tempered martensite + spheroidized carbides
Double tempering (180 °C × 2h × 2)	62-65	5-8	Stable tempered martensite + fine dispersed carbides

The study demonstrates that double tempering at 180 °C for 2 hours (two cycles) achieves the optimal balance of hardness (62-65 HRC), low retained austenite (5-8%), and high dimensional stability.

Wear Resistance and Rolling Contact Fatigue

The study employs ball-on-disk rolling contact fatigue tests and dry sliding wear tests to evaluate performance. The double-tempered condition exhibits 30-40% lower specific wear rate compared to the as-deposited condition, attributed to the elimination of unstable retained austenite and the formation of fine, uniformly dispersed carbides.

Test Condition	Specific Wear Rate (mm³/N·m)	Fatigue Life (cycles to spalling)	Dominant Wear Mechanism
As-deposited	1.8×10⁻⁴	2.5×10⁵	Micro-cracking + spalling
Single temper 180 °C	1.2×10⁻⁴	4.0×10⁵	Abrasive + adhesive
Double temper 180 °C	0.9×10⁻⁴	6.5×10⁵	Mild abrasive
Over-tempered 250 °C	1.5×10⁻⁴	3.0×10⁵	Adhesive + micro-ploughing

Residual Stress Analysis

The as-deposited HVOF cladding layer exhibits high compressive residual stresses (-600 to -800 MPa), which are beneficial for fatigue resistance. However, tempering at temperatures above 200 °C leads to stress relaxation, reducing the compressive stress magnitude. The study recommends maintaining tempering below 200 °C to preserve beneficial compressive stresses while stabilizing the microstructure.

Process and Standards Compliance

For cold-rolling mill roll cladding, the relevant standards include GB/T 12466 (steel for cold-rolling mill rolls), ISO 3977 (bearing steel), and ASTM A285 (bearing steel). The cladding process must be qualified per NB/T 47014, with mechanical property testing including hardness, tensile strength, and impact toughness at the operating temperature.

Requirement	Standard	Acceptance Criteria
Hardness	ASTM E18	62-66 HRC (surface)
Retained austenite	ASTM E1000	< 10% (by XRD)
Compressive residual stress	ASTM E975	> 400 MPa (surface)
Bond strength	ASTM A264	> 95% of base metal tensile
Surface roughness	ISO 4287	Ra ≤ 0.8 μm (after grinding)

Defect Analysis and Countermeasures

Defect	Cause	Countermeasure
Surface cracks	Excessive retained austenite transformation	Double tempering to stabilize austenite
Hardness variation	Inconsistent deposition parameters	Process monitoring and statistical control
Delamination	Poor bond strength	Increase preheat, optimize deposition velocity
Excessive retained austenite	Insufficient tempering	Extend tempering time, lower tempering temperature

Integration with Engineering Practice

In cold-rolling mills, roll surface condition directly affects product quality (surface finish, dimensional accuracy). The study recommends a post-deposition treatment sequence of: tempering at 180 °C for 2 hours, followed by cryogenic treatment at -80 °C for 4 hours, then final tempering at 180 °C for 2 hours. This sequence converts remaining metastable austenite to martensite, achieving hardness above 65 HRC with minimal retained austenite.

For quality control, each roll batch should undergo hardness mapping (minimum 20 points), retained austenite measurement by XRD, and ultrasonic bond strength testing. The study emphasizes that dimensional stability after heat treatment is critical, with maximum allowable distortion of 0.02 mm/m for precision cold-rolling applications.

Study Insights and Implications

The study provides clear evidence that post-weld heat treatment is not merely a residual stress relief step but a critical process variable that determines the long-term performance of GCr15 cladding. The double-tempering approach at 180 °C represents the optimal balance between hardness, toughness, and dimensional stability. For engineering practice, I strongly recommend implementing a cryogenic treatment step between the two tempering cycles to further reduce retained austenite and enhance wear resistance. This three-step treatment protocol (temper-cryo-temper) should be incorporated into the PQR and validated through rolling contact fatigue testing per ISO 281.